It is known in the art relating to engine cylinder deactivation to utilize so-called switching valve lifters responsive to an oil pressure signal to either maintain full operation of all the engine cylinders or deactivate the valves of cylinders having the switching lifters. This cuts out operation of these cylinders as a part of the engine power development process. Currently available switching lifters include a roller cam follower enclosing a hydraulic lash adjuster that is locked in place by a latch pin. When the lifter is in position to operate an associated engine valve, depression of the latch pin by an oil pressure signal allows the valve lifter to telescope within the follower body against the force of a lost motion spring. This allows the valve to remain closed and contribute to deactivation of the associated cylinder.
When the lifter latch pin is in the locked position, a small amount of lash is provided by the clearance hole around the pin. The pin is maintained in a relatively free condition in the hole by the force of a lost motion spring acting against oil pressure in the lash adjuster, which tends to remove lash from the valve actuating mechanism. Because of this, oil supply pressure to the lash adjuster must be maintained below a predetermined maximum pressure in order that the force of the lost motion spring will be adequate to maintain the latch pin in a position that allows unlatching and latching motions free from friction against the follower body. Excessive friction would prevent deactivation and reactivation motions of the latch pin. Accordingly, the cylinder deactivation mode of operation is normally disabled during operating conditions where the engine oil pressure is greater than the specified maximum value, since attempts to deactivate or reactivate the cylinders with excessive oil pressure could result in misoperation and resulting unsatisfactory operation of the engine cylinders.
Under most normal warmed up engine conditions, the system oil pressure fed to the lash adjusters is adequately controlled by an oil pressure relief valve or bypass valve, associated with the engine oil pump, which either recirculates excess oil flow or returns it to the sump in order to maintain a maximum desired oil pressure. However, in some conditions of intermediate engine oil temperatures lower than the normal operating maximum temperatures, high engine speeds may create engine oil system pressures from the engine driven oil pump which exceed the specified maximum pressure and thus prevent use of the cylinder deactivation feature.